Alloy 20 (UNS N08020): The Corrosion-Resistant Alloy for Sulfuric Acid Service
If you handle sulfuric acid, phosphoric acid, or hot contaminated brines, Alloy 20 (Carpenter 20, UNS N08020, W. Nr. 2.4660) is the workhorse iron-nickel-chromium-molybdenum-copper alloy that delivers the best combination of corrosion resistance, weldability, and cost. This guide covers ASTM B463 / B464 / B468 specifications, chemistry, mechanical properties, and real-world service limits in H₂SO₄ from 1% to 98% concentration.
Table of Contents
What is Alloy 20?
Alloy 20 — also called Carpenter 20, Alloy 20Cb-3®, UNS N08020, or Werkstoff Nr. 2.4660 — is an iron-based (Fe ~ 35–40%) austenitic stainless alloy with 20% chromium, 34% nickel, 2.5% molybdenum, and 3.5% copper. The copper addition is the key metallurgical signature — it gives the alloy its remarkable resistance to non-oxidizing acids like sulfuric and phosphoric.
Originally developed by Carpenter Technology in 1935, Alloy 20 was the first “super-austenitic” stainless — predating 904L by 50 years. Today it remains the most cost-effective alloy for handling moderately concentrated sulfuric acid at moderate temperatures — the sweet spot where 316L fails and the more expensive nickel alloys (825, 625, C276) are over-specified.
Chemical Composition & ASTM B464 Plate
ASTM B464 covers plate; B465 covers sheet and strip; B468 covers welded pipe; B463 covers bar and wire. The unified composition limits are:
| Element | Composition (wt%) | Function |
|---|---|---|
| Iron (Fe) | Balance (~35–40%) | Base metal |
| Nickel (Ni) | 32.0 – 38.0 | Austenite stability, reduces SCC risk |
| Chromium (Cr) | 19.0 – 21.0 | Oxidation resistance |
| Molybdenum (Mo) | 2.0 – 3.0 | Pitting & crevice resistance |
| Copper (Cu) | 3.0 – 4.0 | Sulfuric acid resistance (key element) |
| Niobium (Nb) + Ta | 8 × C min, 1.0 max | Carbide stabilization (Cb-3 designation) |
| Carbon (C) | 0.07 max | |
| Manganese (Mn) | 2.0 max | |
| Silicon (Si) | 1.0 max | |
| Phosphorus (P) | 0.045 max | |
| Sulfur (S) | 0.035 max |
The Nb ≥ 8 × C rule is the same as the “stabilized” concept used in 321 stainless — it prevents chromium carbide precipitation at the grain boundaries during welding or service at 425–870°C, which would otherwise cause intergranular corrosion (IGC) in the HAZ.
Mechanical Properties
| Property | Annealed (typical) | Specification Min (ASTM B463) |
|---|---|---|
| Yield Strength 0.2%, MPa (ksi) | 275 (40) | 240 (35) |
| Tensile Strength, MPa (ksi) | 620 (90) | 550 (80) |
| Elongation in 50 mm, % | 45 | 30 |
| Hardness, Brinell (max) | 160 | 217 (per B464) |
| Density, g/cm³ | 8.05 | — |
| Modulus, GPa | 193 | — |
| Max service temp, °C (sustained) | ~400 | — |
| Melting range, °C | ~1,400 – 1,445 | — |
The strength of Alloy 20 is comparable to 304/316 stainless — about 30% lower yield than duplex 2205 but similar to 316L. For most chemical process service, strength is not the design driver; corrosion resistance is.
Corrosion Resistance in Sulfuric Acid
This is what Alloy 20 is famous for. The isocorrosion diagram (0.1 mm/yr lines, mm/y = millimeters per year of metal loss) gives the design envelope:
| H₂SO₄ Concentration | 20°C | 50°C | 75°C | Boiling (~330°C) |
|---|---|---|---|---|
| 1% | < 0.05 mm/y (NR) | < 0.05 (NR) | < 0.1 (R) | 0.3 (R) |
| 10% | < 0.05 (NR) | < 0.1 (R) | 0.2 (R) | 0.8 (use 825 / C276) |
| 30% | < 0.05 (NR) | 0.1 (R) | 0.4 (R) | 1.5+ (use C276) |
| 50% | < 0.1 (R) | 0.2 (R) | 0.7 (use 825) | > 3 (use C276 / 625) |
| 78% (concentrated) | < 0.1 (R) | 0.3 (R) | > 1 (use 825) | > 5 (use C276) |
| 96% (oleum) | 0.2 (R) | > 1 (use C276) | > 3 | > 10 (use Si-Cast Irons) |
NR = Not Recommended. R = Recommended (corrosion rate < 0.5 mm/yr). Numbers are indicative; actual rates depend on aeration, velocity, contaminants (chlorides, fluorides, oxidizers).
The optimum window for Alloy 20 is 10–40% H₂SO₄ below 75°C — exactly the range found in:
- Sulfuric acid alkylation units in refineries (the original application)
- Phosphate fertilizer production (wet-process phosphoric acid, often contains residual H₂SO₄)
- Metal pickling baths
- Sulfur dioxide absorber towers and ducts in sulfuric acid plants
- Galvanizing and anodizing rinse tanks
Other Corrosion Media
- Phosphoric acid (H₃PO₄): excellent resistance — the industry standard for wet-process acid up to 70% concentration and 100°C, including the aggressive “filter acid” with 1–2% fluoride, 1–2% chloride, and entrained gypsum solids.
- Nitric acid (HNO₃): resistant up to ~50% concentration and 80°C — comparable to 304L. Better than 316L.
- Hydrochloric acid (HCl): not recommended above 5% at any temperature. Use Hastelloy B-2 for pure HCl.
- Sodium hydroxide (NaOH): good up to 50% and 80°C; comparable to 316L.
- Seawine / seawater: resistant — comparable to 316L. Not the first choice for hot stagnant chloride (use 2205 or 625 there).
ASTM Product Forms & Specifications
| Product Form | ASTM Specification | Typical Size Range |
|---|---|---|
| Plate, sheet, strip | B463, B464, B465 | 0.5 mm to 100 mm thick |
| Seamless pipe & tube | B729 | 1/2″ to 12″ NPS, schedules 10S–80S |
| Welded pipe | B468, B469 | 1/2″ to 48″ NPS |
| Bar & wire | B473, B474 | Round 6 mm to 300 mm; hex, square, flat |
| Forged fittings & flanges | B462 | 1/2″ to 24″ NPS, 150#–600# |
| Welding fittings | B366 | Butt-weld 90°, 45°, tee, reducer, cap |
| Cast equivalent (Alloy 20 CB-3) | A351 CN7M | Valve bodies, pump casings, impellers |
Note that Alloy 20 is the only iron-nickel-chromium alloy that has its own dedicated ASTM cast grade CN7M — making it the easiest “superaustenitic” alloy to obtain as castings for pumps and valves.
Welding, Forming & Machining
Welding
- Weldable by GTAW, GMAW, SMAW, and PAW — no preheat required on most sections.
- Filler metal: ER320LR (low-carbon, residual-element-controlled) for matching chemistry; or ERNiCrMo-3 (Inconel 625) for dissimilar joints or where the weld will see the most aggressive corrosion.
- Post-weld heat treatment is not required for most applications because the niobium stabilization prevents sensitization.
- For maximum corrosion resistance in the as-welded HAZ, specify a low-carbon heat (≤ 0.03% C) and a solution-anneal (1,000–1,100°C, water quench) — optional but recommended for the most aggressive service.
Forming
- Work-hardens at roughly the same rate as 316 stainless — about twice the rate of mild steel.
- Hot forming: 870–1,150°C, then re-anneal at 1,000°C minimum and water quench.
- Cold forming is the preferred method for most components.
Machining
- Rating: ~25% of B1112 free-machining steel (similar to 316L).
- Use rigid tooling, positive rake, and abundant cutting fluid.
- Carbide inserts (C-2 or C-3 grade) recommended for production work.
Alloy 20 vs 316L vs Hastelloy C276 — When to Choose Each
| Service | 316L | Alloy 20 | Hastelloy C276 |
|---|---|---|---|
| 10% H₂SO₄, 50°C | 1.0 mm/y (fail) | < 0.1 (R) | < 0.05 (R) |
| 30% H₂SO₄, 75°C | > 5 (fail) | 0.4 (R) | 0.2 (R) |
| 78% H₂SO₄, 100°C | Fail | 1.0 (marginal) | 0.3 (R) |
| 30% HCl, 50°C | Fail | Fail | 0.5 (R) |
| Wet H₃PO₄ with F⁻ + Cl⁻ | Fail | < 0.3 (R) | < 0.1 (R) |
| Indicative price vs 316L | 1.0× | ~3.5× | ~9× |
Application Examples
Sulfuric Acid Plants
SO₂ absorber towers, drying towers, coolers, and acid piping in contact-process H₂SO₄ plants. The original application and still the largest user of Alloy 20 worldwide.
Phosphate Fertilizer (DAP/MAP)
Wet-process phosphoric acid evaporators, agitators, attack tanks, filter acid piping. Handles the F⁻ + Cl⁻ + gypsum slurry environment better than 904L for less cost.
Petrochemical Alkylation
Reactor sections, settler internals, acid regenerator piping. The workhorse alloy for H₂SO₄ alkylation since the 1950s.
Pharmaceutical & Fine Chemicals
Reactors and crystallizers handling sulfuric, phosphoric, and mixed organic-inorganic acid streams. The high Ni content gives clean, low-ionic leach rates — meets FDA and USP expectations.
Metal Pickling & Finishing
Pickling baskets, hooks, baskets for sulfuric, mixed acid, and nitric-HF pickling. Replaces lead, rubber-lined steel, and exotic alloys in many lines.
Pump & Valve Castings
ASTM A351 CN7M is the cast equivalent — pumps for H₂SO₄ transfer, valve bodies and trim for acid service, agitator shafts in reactors.
For more on the metallurgy of nickel alloys in chemical service, see our guides on Hastelloy C276 vs C22 and Inconel 625.
Detailed H₂SO₄ Isocorrosion Analysis — Alloy 20 vs 825 vs C276
The isocorrosion diagram for sulfuric acid is the most important single document in an Alloy 20 selection decision. The table earlier in this article gave indicative corrosion rates. Now we present a more detailed comparison across the three most commonly-specified alloys for H₂SO₄ service: Alloy 20, Inconel 825, and Hastelloy C276.
| H₂SO₄ Concentration | Temperature (°C) | Alloy 20 (mm/y) | Inconel 825 (mm/y) | Hastelloy C276 (mm/y) | 316L (for reference) |
|---|---|---|---|---|---|
| 5% | 25 | < 0.05 | < 0.05 | < 0.05 | < 0.1 |
| 5% | 50 | < 0.1 | < 0.05 | < 0.05 | 0.5 (fail) |
| 5% | 80 | 0.3 | 0.1 | < 0.1 | > 5.0 |
| 20% | 25 | < 0.05 | < 0.05 | < 0.05 | 0.3 |
| 20% | 50 | 0.1 | 0.1 | < 0.05 | 2.0 |
| 20% | 80 | 0.5 (marginal) | 0.3 | < 0.1 | > 10 |
| 40% | 25 | < 0.1 | < 0.1 | < 0.05 | > 1.0 |
| 40% | 50 | 0.2 | 0.2 | < 0.1 | > 5.0 |
| 40% | 80 | 1.0 (fail) | 0.5 | < 0.2 | Fail |
| 78% | 25 | 0.1 | < 0.1 | < 0.05 | > 0.5 |
| 78% | 50 | 0.4 | 0.3 | < 0.1 | > 3.0 |
| 78% | 80 | > 1.0 | 0.8 | 0.2 | Fail |
| 96% | 25 | 0.3 | 0.2 | < 0.1 | > 2.0 |
| 96% | 50 | > 1.5 | 1.0 | 0.3 | Fail |
Alloy 20 in Mixed-Acid Environments — The Phosphate Fertilizer Use Case
Wet-process phosphoric acid (WPA) production — the dominant route to phosphate fertilizers (DAP, MAP, TSP) — is the single largest market for Alloy 20 after sulfuric acid alkylation. The WPA slurry is a complex, multi-component corrosive medium: 28–42% P₂O₅ (as H₃PO₄), 2–4% free H₂SO₄, 1–2% HF (as SiF₄, H₂SiF₆), 1–2% chlorides (from phosphate rock), plus entrained gypsum (CaSO₄·2H₂O) solids. This is a “swamp” of different corrosion mechanisms active simultaneously:
- General corrosion by H₂SO₄ — controlled by the Cu content.
- Pitting by chlorides — controlled by Mo content; Alloy 20 is borderline at the high-chloride end of WPA service.
- Crevice corrosion under gypsum scale — the biggest risk for any alloy in WPA. Alloy 20 performs well here because its oxide film reforms faster than 316L.
- Erosion-corrosion by gypsum slurry — Alloy 20’s work-hardening rate (comparable to 316) gives adequate resistance at flow velocities up to ~3 m/s.
- Fluoride attack (HF / F⁻) — Alloy 20 is resistant at the typical 1–2% F⁻ levels in WPA; above 3%, the corrosion rate accelerates and 825 or C276 becomes necessary.
| WPA Service Location | Temperature (°C) | Preferred Alloy | Notes |
|---|---|---|---|
| Reaction (attack) tank | 75–85 | Alloy 20 / 904L | High solids erosion; 20 is well-proven |
| Filter acid piping | 60–70 | Alloy 20 | Lowest cost option that survives the F⁻ + Cl⁻ combo |
| Evaporator tubes (first effect) | 100–120 | 825 / Sanicro 28 | Temperature too high for Alloy 20 |
| Agitator blades | 75–85 | Alloy 20 (cast CN7M) | Excellent erosion resistance in cast form |
| Concentrated acid storage tank | 40–60 | Alloy 20 / 316L | 316L sufficient if free H₂SO₄ < 1% |
| Fluosilicic acid recovery | 50–70 | Alloy 20 | Handles dilute H₂SiF₆ + H₂SO₄ mixture |
Intergranular Corrosion Testing: Why Nb-Stabilization Matters
Alloy 20 is niobium-stabilized (Nb ≥ 8 × C), exactly like Type 321 stainless. This is the single most important metallurgical feature that distinguishes Alloy 20 from non-stabilized super-austenitics like 904L. The purpose of stabilization is to prevent chromium carbide precipitation (sensitization) at grain boundaries during welding or service in the 425–870°C range. Without stabilization, chromium carbides (Cr₂₃C₆) form at boundaries, depleting the adjacent matrix of chromium below the ~12% threshold for passivity — opening a path for intergranular attack (IGA) in corrosive media.
In niobium-stabilized alloys, the Nb preferentially combines with carbon to form NbC, which is stable at high temperatures and does NOT rob the matrix of chromium. The result: Alloy 20 can be welded without post-weld heat treatment and still pass the ASTM A262 Practice E (Strauss Test) or ASTM G28 Method A (Ferric Sulfate – Sulfuric Acid) intergranular corrosion tests — both of which are mandatory for most chemical plant specifications.
| Test Standard | Medium | Temperature | Duration | Purpose |
|---|---|---|---|---|
| ASTM A262 Practice B | Boiling ferric sulfate – 50% H₂SO₄ | Boiling (~120°C) | 120 h | Screening test for sensitized microstructures |
| ASTM A262 Practice E (Strauss) | Boiling copper – copper sulfate – 16% H₂SO₄ | Boiling | 15 h (bend test) | Detects chromium depletion at grain boundaries |
| ASTM G28 Method A | Boiling ferric sulfate – 50% H₂SO₄ | Boiling (~120°C) | 24 h | Standard for nickel alloys (625, 825, C276) |
| ASTM G28 Method B | Boiling 23% H₂SO₄ + 1.2% HCl + 1% FeCl₃ + 1% CuCl₂ | Boiling | 24 h | More aggressive — for high-Mo Ni alloys |
Specification tip: When ordering Alloy 20 for critical chemical service, always include both ASTM A262 Practice E (on the as-received mill product) AND ASTM A262 Practice B (on the welded qualification coupon). The first confirms the base metal was properly solution-annealed from the mill; the second confirms the weld procedure did not sensitize the HAZ. Any Alloy 20 product that fails either test should be rejected.
Cast Equivalent: ASTM A351 CN7M — Pump & Valve Applications
The cast equivalent of Alloy 20 is ASTM A351 Grade CN7M — a 19Cr-29Ni-2.5Mo-3.5Cu grade designed specifically for investment and sand casting of pump casings, valve bodies, impellers, and agitator components. CN7M is the only widely-available cast “super-austenitic” grade listed in ASME B16.34 (valves), ASME B16.5 (flanges), and API 600/602 (gate, globe, and check valves).
| Property | Wrought Alloy 20 (ASTM B463) | Cast CN7M (ASTM A351) |
|---|---|---|
| Chemistry range | Ni 32–38, Cr 19–21, Mo 2–3, Cu 3–4, C ≤ 0.07 | Ni 27.5–30.5, Cr 19–22, Mo 2–3, Cu 3–4, C ≤ 0.07 |
| Yield Strength, MPa (ksi) min | 240 (35) | 170 (25) |
| Tensile Strength, MPa (ksi) min | 550 (80) | 450 (65) |
| Elongation, % min | 30 | 30 |
| Typical casting process | Wrought (plate, sheet, pipe, bar) | Sand casting or investment casting |
| Weld repair | Standard procedure | Allowed with ER320LR, then re-solution-anneal |
| Common NDE | UT (plate), ET (tube), PT (surface) | RT (critical), PT (all), UT (agreed with purchaser) |
The slightly different Ni range in CN7M (27.5–30.5 vs 32–38) is a deliberate concession to castability — lower Ni improves fluidity in the mold. The corrosion resistance is virtually identical to wrought Alloy 20 in most media, though some laboratory tests show a 5–10% higher corrosion rate in CN7M at the upper end of the H₂SO₄ concentration range, attributed to micro-segregation of Cu during solidification. For any casting that will see > 50% H₂SO₄ at > 50°C, request a homogenization anneal (1,150°C / 2h, water quench) after casting to eliminate segregation.
Supply Chain & Inspection: What to Look For When Receiving Alloy 20
Alloy 20 is widely stocked at distributors and service centers worldwide, but it is also one of the most commonly mis-identified alloys in the supply chain. Because 316L and Alloy 20 look identical to the naked eye (both are non-magnetic, silver-gray, and austenitic), PMI (Positive Material Identification) is mandatory on every incoming piece. Here is the receiving inspection checklist:
- Visual check: Confirm the heat number (HT#) stenciled on each piece matches the MTC. Reject any piece with illegible or missing markings.
- PMI by portable XRF (handheld): Verify that the alloy type is N08020 or at minimum shows Ni > 30%, Cr ~20%, Mo ~2.5%, Cu ~3.5%. A 316L piece will show Ni ~10–14% and Cu = 0 — that is an immediate rejection flag. Note that handheld XRF cannot reliably measure C, Nb, or N — those require OES or LECO combustion for full verification.
- MTC review: Check that every element in the ASTM B463 table is reported with an actual value (not “≤ max”), that the certificate number is unique, that the testing laboratory is ISO/IEC 17025 accredited, and that the signature/stamp of the mill’s authorized inspector is present. A generic “Typical Chemical Analysis” sheet is not acceptable for critical service.
- Intergranular corrosion test report: If ASTM A262 Practice E or ASTM G28 Method A was specified, verify that the test report is attached and the result is “Pass.”
- Dimensional check: Measure thickness, width, length, and straightness against the ASTM tolerance tables. Alloy 20 is often ordered in small quantities and sometimes pieces from different heats are mixed in the same bundle.
- PMI on welds (if applicable): If the piece is a shop-welded pipe or fitting, PMI the weld metal to confirm it is ER320LR — not ER316L or ER308L misapplied by the fabricator.
Alloy 20 Failure Analysis: Common Field Failures & Root Causes
Despite its excellent track record in sulfuric acid service, Alloy 20 can fail under specific conditions that are not always obvious at the design stage. Understanding these failure modes helps procurement and maintenance teams avoid them:
Failure Mode 1: Weld HAZ Attack in Non-Stabilized Heats
If a heat was produced with insufficient niobium (Nb < 8 × C) — either by error or by using an older, non-Cb-3, non-UNS N08020 composition — the HAZ becomes sensitized during welding. In sulfuric acid or nitric acid service, intergranular attack follows the HAZ like a knife cut, leading to through-wall penetration within weeks. Root cause: accepting material without Nb verification on the MTC. Prevention: always verify Nb ≥ 8 × C on each heat certificate; reject any heat that fails this check.
Failure Mode 2: Chloride Pitting at Gasket Seats
In a sulfuric acid alkylation unit, Alloy 20 flange faces developed deep pits at the inner edge of the gasket seating surface after ~3 years. Investigation revealed that the alkylation acid contained ~200 ppm chlorides from crude oil carryover, and the stagnant zone under the gasket concentrated chlorides by evaporation during shutdowns. Root cause: Alloy 20’s PREN of ~28 is insufficient for stagnant chloride concentration above ~1,000 ppm. Prevention: specify PTFE envelope gaskets (not compressed fiber) to eliminate the crevice, or upgrade to 825 (PREN ~31) / 625 (PREN ~50) for the flange facings.
Failure Mode 3: Velocity-Accelerated Corrosion in Slurry Service
In a phosphoric acid plant, an Alloy 20 pipe elbow in gypsum slurry service failed by through-wall perforation at the outer radius after 18 months — while straight sections at the same temperature and chemistry were untouched after 10 years. Root cause: erosion-corrosion at the elbow, where slurry impact velocity exceeded the 3 m/s threshold for Alloy 20. Prevention: use long-radius elbows (≥ 3D), increase wall thickness by 1 schedule, or upgrade to a harder alloy (CD4MCuN duplex for WPA slurry is common).
Failure Mode 4: Contamination from Carbon Steel Tooling
Newly fabricated Alloy 20 vessels showed rust spots and shallow pitting before they ever entered service. Investigation found that the shop had used the same carbon-steel wire brushes, grinding discs, and lifting chains for Alloy 20 as for carbon steel fabrication — embedding iron particles in the surface. In the humid coastal air, the iron particles rusted, creating a galvanic cell that initiated pitting on the Alloy 20 surface. Root cause: inadequate shop segregation of stainless and nickel alloy fabrication from carbon steel. Prevention: dedicated tooling, pickling + passivation after fabrication (ASTM A380), and a ferroxyl test to detect free iron on the surface before shipment.
Pickling & Passivation of Alloy 20 — ASTM A380 Requirements
After fabrication — especially after welding, hot forming, or thermal cutting — Alloy 20 surfaces must be pickled and passivated to restore the optimum corrosion-resistant surface condition. The standard reference is ASTM A380 / A967 (Chemical Passivation Treatments for Stainless Steel Parts) adapted for nickel alloys. The procedure:
- Degrease: Remove all oil, grease, cutting fluid, and marking ink with an alkaline cleaner or solvent. Any organic residue will shield the surface from the pickling acid and create an untreated zone.
- Pickle: Immerse in a nitric-hydrofluoric acid solution (typically 10–20% HNO₃ + 1–3% HF at 50–60°C for 10–30 minutes) to dissolve the heat tint (oxide scale) and a thin layer of the underlying chromium-depleted metal. For large vessels, pickling paste (gel-form) is applied to weld seams and HAZ. Do NOT use hydrochloric-acid-based pickling — HCl will pit Alloy 20.
- Rinse: Thorough water rinse (deionized or low-chloride) to remove all residual acid. Any acid trapped in crevices, under gaskets, or in dead legs will concentrate during service and cause corrosion.
- Passivate: Immerse in 20–25% nitric acid at 50–60°C for 20–30 minutes (ASTM A967, Nitric 3 method) to rebuild the chromium-rich passive film. This step is essential — the passive film on freshly pickled Alloy 20 is incomplete and vulnerable to pitting until passivation restores the full Cr/Ni oxide layer.
- Final rinse and dry: Deionized water rinse, followed by hot-air drying. The passivated surface should show a uniform bright-silver appearance with no water stains or “flash rust.”
- Verification: Perform a ferroxyl test (ASTM A380) to confirm the absence of free iron contamination, or a copper sulfate test (ASTM A967 Practice C) for passivation completeness on each lot of fabricated items.
Cost-Benefit Analysis: Alloy 20 vs Alternatives in H₂SO₄ Service
For procurement teams, the choice between Alloy 20, 825, and C276 is fundamentally an economic decision once corrosion resistance is confirmed. Here is a worked example for a typical sulfuric acid plant project:
| Scenario: 3 km of 4″ Sch 40 piping, 40% H₂SO₄, 60°C | Alloy 20 (N08020) | Inconel 825 (N08825) | Hastelloy C276 (N10276) |
|---|---|---|---|
| Material cost (pipe + fittings + flanges) | $180,000 | $310,000 | $840,000 |
| Fabrication & welding | $85,000 | $95,000 | $110,000 |
| NDE + PMI + IGC testing | $15,000 | $18,000 | $20,000 |
| Installation + commissioning | $60,000 | $60,000 | $60,000 |
| Total CAPEX | $340,000 | $483,000 | $1,030,000 |
| Expected service life (40% H₂SO₄, 60°C) | 15–20 years | 25+ years | 40+ years |
| Corrosion allowance (mm) | 1.5 | 0.5 | 0.25 |
| Project life requirement | 20 years | 20 years | 20 years |
| Meets project life? | Yes | Yes | Yes (overqualified) |
| Relative cost vs cheapest acceptable | 1.0× | 1.4× | 3.0× |
For the typical 40% H₂SO₄ at 60°C scenario — the most common in sulfuric acid alkylation — Alloy 20 is the clear economic winner. The 1.4× premium for 825 is only justified if the client requires a 30+ year service life with zero corrosion allowance, or if the acid contains > 500 ppm chlorides (where 825’s higher Mo gives a margin over Alloy 20). C276 at 3× the cost is almost never justified at this service condition unless the acid contains reducing contaminants (e.g., HCl carryover from crude desalting) that attack the Mo levels in both 20 and 825.
Additional Frequently Asked Questions
Can Alloy 20 be used in nitric acid (HNO₃) service?
What is the difference between Alloy 20 and Carpenter 20Cb-3?
Is Alloy 20 magnetic?
Does Alloy 20 require post-weld heat treatment?
How to Select an Alloy 20 Supplier
When sourcing Alloy 20 for chemical plant or oil & gas service, evaluate potential suppliers on these criteria:
- Mill traceability: Can the supplier provide the original mill certificate (not a distributor copy)? The MTC must name the producing mill (e.g., VDM, Outokumpu, Special Metals, Nippon Steel) and the heat number.
- ISO/IEC 17025 accreditation: Is the testing lab on the MTC accredited to ISO/IEC 17025? If not, request an independent re-test by an accredited lab before acceptance.
- Stock rotation: How long has the material been in stock? Alloy 20 does not degrade with age, but long-stored material may have surface contamination, lost traceability, or obsolete test reports. Preferred: material less than 2 years from the mill test date.
- PMI capability: Does the supplier perform 100% PMI on outgoing shipments? A reputable supplier PMI-tests every piece — not just a statistical sample — and stamps the PMI result on the material.
- Packaging: Is the material protected from iron contamination during transport? Preferred: individual piece wrapping, wood/polymer dunnage, and plastic end-caps on pipe with dessicant inside.
- After-sales support: Can the supplier provide a metallurgist for consultation if the material fails a receiving inspection or if welding issues arise?
Frequently Asked Questions
What is the difference between Alloy 20 and Alloy 20Cb-3?
Can Alloy 20 handle hot concentrated sulfuric acid?
Is Alloy 20 the same as Sanicro 28 or 904L?
What welding rod is used for Alloy 20?
Does Alloy 20 pit in chloride solutions?
What is the standard material test certificate for Alloy 20?
Source Certified Nickel Alloys — Mill Direct, EN 10204 3.1/3.2
Inconel, Incoloy, Hastelloy, Monel, Alloy 20, Duplex — full bar, sheet, plate, tube & pipe inventory with full traceability. Fast global shipping.
